Resumo:
O magmatismo granítico no domínio da Zona Transversal da Província Borborema, NE do Brasil, ocorreu em três intervalos de tempo principais: 650- 620 Ma, 590-570 Ma e 545-520 Ma. O intervalo mais antigo é caracterizado por intrusões de granitóides sin- cinemáticos cálcio-alcalinos de alto K com epidoto magmático, cálcio-alcalinos e shoshoníticos. O segun - do intervalo de tempo é marcado por intrusões abun - dantes de magmas cálcio-alcalinos de alto K sem epi - doto magmático, além de magmas sieníticos peral - calinos, metaluminosos de alto K, ultrapotássicos e raros shoshoníticos. Magmas peralcalinos e do tipo A marcam o final do ciclo Brasiliano na região. Os gra - nitóides mais antigos apresentam valores de TDM mais jovens (< 2.0 Ga) e valores menos negativos de εNd (cerca de -2 a -14) comparados com aqueles para os granitóides de 590-570 Ma, para os quais os valores de TDM variam de cerca de ca 1,5 a 2,5 Ga, e os de
ε
Ndde cerca de -8 a -20. Os valores mais altos e variáveis das razões iniciais 87Sr/86Sr são observados nos grani - tóides de 650-620 Ma. Esses dados indicam uma mudança importante da fonte do magma com o tempo geológico. Os granitóides mais antigos parecem repre - sentar uma mistura de magma derivado do manto juve- nil, neoproterozóico, com magma derivado de fusão de crosta continental paleoproterozóica, o que produziria idades modelo Nd intermediárias, de cerca de 1,2 a 1,4 Ga. Isto é compatível com a mistura de magmas deriva - dos do manto e crosta, como indicado por relações de campo que indicam um forte relacionamento entre mag - mas granitíticos e dioríticos. Os plútons de 590-570 Ma apresentam valores isotópicos de Sr e Nd compatíveis com magmas formados por refusão de crosta continen - tal inferior paleoproterozóica, o único reservatório geoquímico com aquelas características. A única exceção entre as rochas dessa idade são os sienitos peralcalinos ultrapotássicos, que apresentam razões iniciais de Sr alta e valores muito negativos de
ε
Nd“values”, interpretados como derivados de uma fonte no manto enriquecida em elementos incompatíveis. O grupo mais antigo de rochas apresenta assinaturas geo - químicas típicas de granitos sin-tectônicos, enquanto que o grupo de 590-570 Ma mostra assinaturas típicas de magmas tardi-a pós-tectônicos.
Palavras-chave: Granitos, Granitos Cálcio-Alcalinos, Sienitos Peralcalinos, Granitos Shoshoníticos, Epidoto INTERMEDIATE TO ACIDIC MAGMATISM AND CRUSTAL EVOLUTION IN THE TRANSVERSAL ZONE, NORTHEASTERN BRAZIL
Valderez P. Ferreira NEG-LABISE, Departamento de Geologia, UFPE, Recife, PE
valderez@ufpe.br Alcides N. Sial NEG-LABISE, Departamento de Geologia, UFPE, Recife, PE
ans@ufpe.br Márcio M. Pimentel Instituto de Geociências, UnB, Brasília, DF
marcio@unb.br Cândido A.V. Moura Centro de Geociênciss, UFPA, Campus do Guamá, Belém, PA c_moura@ufpa.br
Magmático, Evolução Crustal, Zona Transversal, Província Borborema.
Abstract:
Granitic magmatism in the Transversal Zone domain of the Borborema province, NE Brazil, occurred in three main time intervals: 650-620 Ma, 590-570 Ma and 545-520 Ma.
The oldest is characterized by intrusions of syn-kinematic magmatic-epidote bearing high-K calc-alkalic, calc-alkalic and shoshonitic granitoids. The second time interval is marked by abundant intrusions of magmatic epidote-free high-K calc-alkalic magmas, as well as peralkalic, metalumi- nous high-K syenitic, unique ultrapotassic, and rare shoshonitic magmas. Peralkalic and rare A-type magmas mark the end of the Brasiliano cycle in the region. The oldest granitoids present younger TDM(< 2.0 Ga) and less negative
ε
Nd (ca. -2 to -14) compared with the 590-570 Ma granitoids, for which TDMvaries from ca 1.5 to 2.5 Ga, andε
Nd fromca. -8 to -20. The highest and more variable initial 87Sr/86Sr values are observed in the 650-620 Ma granitoids. These data indicate an important change in the magma source with time.
The oldest granitoids seem to represent a mixture of Neoproterozoic, juvenile, mantle-derived material with magma derived from the melting of Paleoproterozoic conti- nental crust, which yielded intermediate Nd model ages, of ca. 1.2-1.4 Ga. This is compatible with the mixture of mantle- derived and crustal magmas, as indicated by field relation- ships that show close relationships between host granitoid and K-dioritic magmas. The 590-570 Ma plutons have Sr and Nd isotopic values compatible with magmas formed by remelting of Paleoproterozoic lower continental crust, the only geochemical reservoir with these characteristics. The only exception among these rocks are the ultrapotassic peral- kalic syenitoids, which present high initial Sr ratios and very negative
ε
Nd values, interpreted as derived from an incom- patible element enriched mantle source. The oldest group of rocks has signatures typical of syn-tectonic granites, while the 590-570 Ma group has signatures typical of late- to post- tectonic magmas.Keywords: Granites, Calc-Alkalic Granites, Peralkalic Syenites, Shoshonitic Granites, Transversal Zone, Magmatic Epidote, Crustal Evolution, Borborema Province.
Resumen:
Magmatismo granítico en la Zona Transversa de la Província estructural de Borborema, en el nordeste de Brasil, se produjo en tres intervalos de tiempo: 650-620 Ma, 590-570 Ma y 545-520 Ma. El grupo más viejo está caracterizado por intrusiones de rocas graníticas calco-alcalinas con alto pota- sio, calco-alcalinas y shoshoníticas, sin a tardi-cinemáticas, con epidota magmática. El segundo intervalo de tiempo de intrusiones está marcado por abundantes magmas calco- alcalinos de alto potasio sin epidota magmática, sienitas per- alcalinas o metaluminosas de alto potasio, magmas ultra- potásicos y, raramente, shoshoníticos. Magmas peralcalinos y escasos magmas del tipo-A marcan el final del ciclo Brasiliano en esta región. Los granitos más viejos presentan TDM más joven (< 2.0 Ga) y valores menos negativo
ε
Nd(desde -2 hasta -14) comparados con los granitos con 590- 570 Ma para los cuales TDMvaría desde 1.5 hasta 2.5 Ga, y
ε
Nd desde ca. -8 hasta -20. Valores más altos y más variables de razones iniciales 87Sr/86Sr son observados en el grupo de granitoides con 650-620 Ma. Estos datos indican un impor- tante cambio de fuente de magma con el tiempo. Los grani- toides más viejos parecen representar una mezcla de material joven derivado del manto, en el Neoproterozoico, con magma derivado de la fusión de la corteza continental Paleoproterozoicaque ha generado edades modelos intermediarias desde 1.2 hasta 1.4 Ga. Esto es compatible con un proceso de mezcla de magmas derivados de la corteza, como lo indican las rela- ciones de campo que muestran íntima relación entre magmas graníticos y magmas dioríticos potásicos. Los plutones con 590-570 Ma tienen valores isotópicos de Sr y Nd compatibles con magmas formados por refusión de la corteza continental Paleoproterozoica inferior, el único reservatorio con estas características. La excepción entre estas rocas corresponden a las sienitas peralcalinas ultrapotásicas que presentan alta razón isotópica inicial de Sr y valores de
ε
Nd muy negativos, interpretados como derivados de una fuente en el manto muy rica en elementos incompatibles. El grupo de rocas graníti- cas más viejas posee signaturas geoquímicas típicas de gra- nitos sintectónicos, mientras los granitos más jóvenes pre- sentan signaturas típicas de magmas tardías pos-tectónicos.Palabras llave: Granitos, Granitos Calco-Alcalinos, Sienitas Peralcalinas, Granitos Shoshoníticos, Epidota Magmática, Magmas Ultrapotásicos, Zona Transversa, Evolución Cortical, Província Borborema.
Introduction
The Borborema structural province, northeastern Brazil, is a mosaic of metasedimentary fold belts and massifs separated by a complex system of continental-scale strike-slip shear zones (Almeida et al., 1981), which together with the voluminous granitic magmatism, are the most outstanding features of this province (Caby et al., 1991).
The tectonic evolution of the province has been explained by models involving the accretion of tectonostrati- graphic terranes as the major mechanism of crustal growth (eg. Brito Neves et al., 1995; Santos, 1996; Santos et al., 1997, Santos & Medeiros, 1999; Brito Neves et al., 2000).
Mafic to ultramafic complexes, some of them interpreted as oceanic crust remnants (Beurlen et al., 1992), occur nearby suspect terrane boundaries, and were used by Santos et al.
(1997) as argument to reinforce the hypothesis of collision of continental blocks. Juxtaposition of blocks during the Brasiliano cycle has been invoked by many authors, on the basis of structural geology (eg. Bittar & Campos Neto, 2000), U-Pb zircon ages and Sm-Nd isotopic data from orthogneisses (e.g.Fetter et al., 2000), as well as from undeformed granitic rocks (eg.Sampaio et al., 2000; Silva Filho et al., 2000).
This process of terrane collage took place during assembly of the western Gondwana (Santos & Brito Neves, 1993; Brito Neves et al., 2000). Some authors, however, pres- ent alternative hypothesis to the generally accepted model of terrane accretion, based on geochemical signatures of high-K calc-alkalic and associated dioritic rocks (e.g.Mariano et al., 2000; Neves et al., 2000), as well as on the lack of some parameters indicative of continental margin, such as ophio- lites (Neves, et al., 2000; who proposed an intracontinental orogen for the whole province).
In the present study, the geochemical and isotopic char- acteristics of the intermediate to acidic magmatism intrusive during the Brasiliano orogeny in the Transversal zone domain of the Borborema Province, will be reviewed and discussed, aiming to understand their meaning in the scenario of crustal evolution of the region.
Geological setting
The Borborema province is characterized by a gently- dipping regional foliation observed in both supracrustal and basement rocks (Caby et al., 1995), and by a network of con- tinental scale transcurrent ductile shear zones (Vauchez et al.,
alkalic, and shoshonitic granitoids at 650-620 Ma.
(2) a late to post-collisional period (590-570 Ma), marked by relative movements of terranes partially amalga- mated, and migration of terranes along E-W Pernambuco and Patos mega shear zones, as well as along the narrower NE- trending shear zones connecting these major shear zones, as indicated by the synkinematic crystallization of several plu- tons emplaced along these shear zones. This period is charac- terized by intrusions of voluminous magmatic epidote-free high-K calc-alkalic magmas, peralkalic, metaluminous high- K syenitic, and rare shoshonitic magmas, as well as unique ultrapotassic magmas, until most movements along shear zones stopped.
Peralkalic and rare A-type magmas mark the end of the Brasiliano cycle in the region.
The Transversal Zone domain
The Transversal Zone domain of the Borborema province has been interpreted as a superterrane (Santos, 1996;
Santos et al., 1997) assembled after a collage of several ter- ranes and belts, which are from east to west: Granjeiro, Cachoeirinha (Brasiliano age), and the pre-Brasiliano ter- ranes Riacho Gravatá, Alto Pajeú, Alto Moxotó and Rio Capibaribe (Fig. 1). The consolidation of the pre-Brasiliano terranes is related to the Transamazonian and Cariris Velhos cycles, but they have been reworked during the Brasiliano cycle (Santos et al., 2003).
A complete picture of the tectonic setting and crustal evolution of the Transversal Zone is still far from being solved and consensual, but geochronological data, including Rb-Sr, zircon TIMS and SHRIMP U-Pb on a significantly large num- ber of granitic plutons, led Brito Neves et al. (2003) to propose three main stages of granitic magmatism in the province: 650- 625 Ma, 580-570 Ma e 545-520 Ma.
Almeida et al. (1967) were pioneers in the study of gran- ite types in this part of the Borborema province. They described in the Granjeiro and Cachoeirinha terranes, on the basis of pet- rography, the granitoids of the so-called Itaporanga and Conceição types, terminology that is still used in the region.
Eight petrogenetic suites were described in the Transversal Zone by Sial (1987) and Ferreira et al. (1998), including the two types recognized by Almeida et al (op cit.), on the basis of petrography and mineralogy, chemistry, and 1995). These Neoproterozoic shear zones are believed to have
developed within a continental plate to accommodate the deformation imposed by oblique collision active at the mar- gin (Vauchez et al., op cit.).
The structural characteristics and rock types of the Borborema province were developed mainly during the late Mesoproterozoic to early Neoproterozoic (Cariris Velhos event) and late Neoproterozoic-Cambrian (Brasiliano/Pan African orogeny) (Brito Neves et al., 2000; 2003). The Brasiliano cycle (670-570 Ma) promoted reworking of the crust formed during the earlier Cariris Velhos event and was the major tectonothermal event in the Borborema province.
This is attested by the structures and by a large volume of granitic magmatism (~30% of the exposed rocks) that was added to the crust during this event. The Cariris Velhos-age rocks have been observed only in the Transversal Zone domain of the Borborema province (Brito Neves et al., 2000).
Two of the shear zones in the province, the E-W-trend- ing dextral Pernambuco and Patos shear zones, stand out for their large dimensions, which reach over 500 km and 2 km in width, linked with narrower (typically 1 km) N- NE-trending shear zones (Neves, 1996). They subdivide the province into three major domains (Fig. 1): (a) setentrional domain, north of the Patos shear zone, (b) Transversal zone domain, between the two shear zones and (c) the meridional domain, south of the Pernambuco shear zone. These domains are inter- preted as representing a collage of smaller lithotectonic domains (Brito Neves et al., 2000). Spatial association of granitic plutons and shear zones, as well as peralkalic syenitic dike sets parallel to or slightly oblique to the shear zone, has led many authors to believe that shear zones have controlled the magma ascent and emplacement (eg. Ferreira & Sial, 1986; Neves, 1991; Jardim de Sá, 1993; Archanjo et al., 1994;
Neves & Vauchez, 1995; Araújo et al., 1995; Hollanda et al., 1998). However, regional deformation, magma emplacement and shear zone development are interpreted as successive events in the Transversal Zone (Neves & Vauchez, 1995).
Integration of geochronological, geochemical and field data, on both granitic and regional metamorphic rocks, sug- gest two major episodes in the Brasiliano cycle:
(1) collisional period (670-620 Ma), characterized by low-angle foliation, and by intrusion of abundant syn-kine- matic magmatic-epidote bearing high-K calc-alkalic, calc-
Fig. 1. Transversal Zone of the Borborema structural province, northeastern Brazil, emphasizing Neoproterozoic granitic and syenitic plutons. Terranes are named after Santos et al.(1997) - Zona Transversal da província estrutural da Borborema, Nordeste do Brasil, enfatizando os plútons neoproterozóicos graníticos e sieníticos. Os terrenos são nomeados de acordo com Santos et al.(1997)
isotopic characteristics, four of which are magmatic epidote- bearing (mEp). These are: mEp metaluminous high-K calc alkalic, metaluminous high-K calc alkalic epidote-free, mEp trondhjemitic, mEp metaluminous calc-alkalic, mEp shoshonitic, peralkalic, peralkalic ultrapotassic, and metalu- minous high-K syenitoids. Further work focussing the Alto Pajeú terrane, confirmed this grouping (eg.Guimarães et al., 1998). One A-type calc-alkalic anorogenic complex is described in the Alto Moxotó terrane (Melo et al., 1996).
Large areas of the Granjeiro terrane are covered by Mesozoic sediments of the Araripe Basin and are not well known. Only a few plutons are described in this terrane.
Among them are the 580 Ma magmatic epidote-free high-K calc-alkalic granitoids (Itaporanga, Bodocó and Serra da Lagoinha granitoids) and peralkalic syenitic, foliated plutons, emplaced next to the boundary with the Cachoeirinha terrane.
The Cachoeirinha terrane corresponds to a late Neoproterozoic foldbelt that contains low-grade psammitic and pelitic rhythmite beds with a few intercalations of mafic to fel- sic volcanic rocks (Brito Neves et al., 2000). This terrane used to be described together with the Riacho Gravatá terrane as a single belt, and known as the Piancó-Alto Brígida (Brito Neves, 1983) or as the Cachoeirinha-Salgueiro foldbelt (Sial, 1984). The Brasiliano magmatism during the 650-620 Ma interval, in this terrane, is mainly calc-alkalic (e.g.the plutons regionally known as Conceição-type granitoids), with subordi- nate throndhjemite (e.g. Serrita pluton), whereas during the 590-570 Ma interval rocks are more K-enriched, shoshonitic to peralkalic in composition (e.g.Serrote dos Cavalos stock).
The Riacho Gravatá belt comprises volcanic rocks (mainly felsic, with subordinate mafic and intermediate) with minor sedimentary rocks, metamorphosed in greenschist facies. Brasiliano granitoids are rare in this domain.
The Alto Pajeú terrane is a metavolcano-sedimentary domain with peak metamorphic age around 960 Ma, during the contractional Cariris Velhos event (Santos et al., 2003).
This terrane was deformed once more during the Brasiliano cycle, which in this domain is characterized by voluminous magmatism in three time intervals, beginning with high-K calc-alkalic and shoshonitic magmas, intruded during the 650-620 Ma interval (e.g. Tavares batholith, Pessôa, 2001;
Conceição das Creoulas batholith, Brasilino et al., 1998).
Magmatism in the 590-570 Ma interval began with high-K calc-alkalic series followed by unique ultrapotassic magmas (eg. Triunfo syenite, Ferreira et al., 1994) so far not found in any other domain of the Borborema province. Peralkalic dikes cutting across the boundary between this and the Cachoeirinha and Rio Capibaribe terranes (Silva Filho et al., 1993; Ferreira & Sial, 1997; 2002) register the end of the Brasiliano cycle (520 Ma) in the Borborema province.
The Rio Capibaribe terrane consists of Transamazonian (2.2-1.8 Ga) basement covered by two major complexes. The oldest is the Vertentes complex that encompasses a migma- tized volcano-sedimentary sequence probably related to the Cariris Velhos event (Brito Neves et al., 2000). The younger sequence (Surubim complex) is mainly composed of metapelites and metacarbonates of probable Neoproterozoic age. Mesoproterozoic anorogenic magmatism (gabbro, anorthosite, mafic dikes and A-type granites) is a characteris- tic of this terrane (Brito Neves et al., op cit.). Neoproterozoic granitic magmatism in this terrane started with high-K calc- alkalic magmas at ca. 690 Ma (Bezerros pluton; Silva et al., 1996). The peak of magmatism at 580 Ma was mostly of met- aluminous high-K (eg. Bom Jardim syenite; Guimarães and Silva Filho, 1997) to high-K calc-alkalic rocks (e.g. Fazenda Nova and Serra da Japecanga complexes, Neves & Vauchez, 1995). Post-kinematic magmatism is characterized by silica- oversaturated peralkalic intrusions (e.g. Moderna pluton, Ferreira & Sial, 2002).
The Alto Moxotó terrane consists of large areas of Paleoproterozoic basement, including some Archean rem- nants, and some early Neoproterozoic supracrustal sequences (Lagoa das Contendas, Sertânia, Caroalina complexes; Brito Neves et al, 2000). Brasiliano granitic magmatism is rare in this terrane compared to other terranes, but it contains one of the two A-type granitic complexes described in the Transversal Zone (Prata complex, Melo et al., 1996).
Fig. 2. QAP diagram for neoproterozoic granitic and syenitic plutons of the Transversal Zone. The magma series trends are from Lameyre and Bowden (1982). TH = trondhjemite, LKCA = low-K calc-alkalic, MKCA = medium-K calc-alkalic. Shaded areas represent modal com- positions for the studied granitoids and syenitoids, classified according to one of these series
- Diagrama QAP para plutons Neoproterozócios graníticos e sieníticos da Zona Transversal. As séries de magmas são de Lameyre e Bowden (1982). TH = trondhjemito, LKCA = cálcio-alcalino de baixo K, MKCA
= cálcio-alcalino de médio K. As áreas sombreadas representam as composições modais dos granitóides e sienitóides estudados, classifica- dos de acordo com uma dessas séries
Fig. 3. Amphibole-rich clots hosted by diorite enclave, which is in turn hosted by 650-620 Ma magmatic epidote-bearing gran- odiorites, in the Cachoeirinha-Salgueiro terrane, Transversal Zone - Agregados ricos em anfibólios inclusos em enclave diorítico, que por sua vez está incluso em granodioritos com epidoto magmático de 650-620 Ma, no terreno Cachoeirinha-Salgueiro, Zona Transversal
bole, biotite, microcline, epidote, and sphene, a mineral com- position typical of I-type granitoids, as defined by Chappell &
White (2001). Unlike the high-K calc-alkalic (HKCA) grani- toids in the Transversal Zone, megacrysts in these CA plutons are plagioclase. Two plutons, Santo Antonio and Angico Torto, exhibit kyanite-bearing black-spotted aureoles a few meters from the contact with these plutons, characterized by fine-grained mica foliation, and the assemblage garnet, kyan- ite, staurolite, muscovite with Si < 3.1, biotite, plagioclase and quartz. Quartz and rutile inclusions in garnet suggest peak P.9 kbar during garnet growth (Caby & Sial, 1996)
Amphibole-rich clots are common and widespread in all plutons of this kind in the Cachoeirinha terrane (Fig. 3).
They consist of deep green calcic amphibole aggregates frac- tionated from host magma, or as fine-grained, angular amphi- bolite containing calcic plagioclase that is regarded as frag- ments from the source rock for the tonalite-granodiorite mag-
Fig. 4. Photomicrographs of magmatic epidote in 650-620 Ma granitoid from the Transversal Zone: (a) euhedral, 1 mm long, partially enclosed by biotite (nicols crossed). (b) subhedral (ca. 1.2 mm long) with allanite core, partially enclosed by biotite (nicols not crossed)
- Fotomicrografias de epidoto magmático em granitóides de 650-620 Ma da Zona Transversal: (a) euédrico, 1 mm de comprimento, parcialmente englobado por biotita (nicóis cruzados); (b) subédrico (ca de 1.2 mm de comprimento) com núcleo de allanita, parcialmente englobado por biotita (nicóis paralelos)
a
b
Field relationships and petrography
650-620 Ma-old, syn-kinematic magmatism:
Magmatic epidote-bearing granitoids
Magmatic epidote (mEp)-bearing calc-alkalic (CA) granitoids in the Cachoeirinha terrane are metaluminous to slightly peraluminous tonalites to granodiorites, forming batholiths and stocks that constitute the so-called Conceição- type granitoids, as described by Almeida et al. (1967). They intruded low-grade metaturbidites and metapelites (Sial, 1993; Sial et al., 1999). All intrusions have crystallization ages around 630 Ma. They form a compositional trend in the QAP diagram following the low-K calc-alkalic trend of Lameyre & Bowden (1982) (Fig. 2). They usually form equigranular, medium- to coarse-grained, round to elongate plutons composed of zoned plagioclase, quartz, calcic amphi-
ning and inclusions of zircon that display inherited cores.
Some primary epidote grains have allanite core.
The HKCA plutons in this terrane are porphyritic monzodi- orites to granodiorites characterized by K-feldspar megacrysts, up to 15 cm long. Magmatic epidote in these plu- tons occupies up to 5% per volume and is included in plagio- clase or rimmed by biotite, sometimes with allanite core.
They present magmatic structures indicative of high fluid dynamics and minor mingling with K-dioritic magmas.
These structures are observed in all plutons of this type in this terrane, but the Tavares and Brejinho plutons, in particular, are the ones where magmatic structures are very well devel- oped and preserved. Among the structures (Fig. 5) are (Pessôa, 2001): (a) elliptical and circular structures character- ized by rythmic, concentric banding, in which mafic and fel- sic bands alternate, in a pattern observed in vertical and hori- zontal expositions, indicating a tridimensional nature; (b) Ladder dikes, and up a 1 m long snail structures, in which magma flow is at a high angle to the magmatic foliation. They are characterized by concave mafic-rich bands that alternate with felsic-rich bands, and mineral composition similar to the host granite, and in many of them K-feldspar cumulates occur in the nucleus; (c) “channel” composed of pillow-like K dior- ites sitting in a felsic matrix. These structures are indicative of convection during the formation of the magma chamber due to intermittent heat input from external source. These fea- tures are interpreted to be due to convective currents origi- nated by underplating of mafic magma (Weinberg et al., 2001), implying in contrast of temperatures within the mas (Sial, 1993; Sial et al., 1999). High δ18O (zircon) values
(11.1‰-11.8‰) are found in the Emas granitoid, one pluton of this type in this terrane (Ferreira et al., 2003). These high magmatic values are compatible with altered seafloor basalt, which has suffered oxygen exchange with low-temperature oceanic water, as protolith. Amphibole-rich clots showing high δ18O (whole-rock) values (9.9‰-11.5‰), usually 1.5‰
lower than in corresponding host granitic rocks (Sial et al., 1998) support this hypothesis.
Several plutons in the Alto Pajeú terrane contain mag- matic epidote (Sial et al., 1999) (Fig. 4): Teixeira granitoid (shoshonitic), Brejinho, Tavares, Conceição das Creoulas, Murici, and Riacho do Icó plutons (metaluminous high-K calc alkalic).
The E-W-trending Teixeira composite batholith is formed by three intrusions: (a) shoshonitic monzonite (cen- tral-eastern part), (b) throndhjemitic tonalite (western por- tion), and (c) peralkalic syenite (a small eastern intrusion).
The shoshonitic granitoids predominate over the other types.
They are mostly equigranular, fine- to medium-grained pera- luminous quartz monzonites to monzogranites, forming a trend between that of the high-K and medium-K calc alkalic series in the QAP diagram (Fig. 2). In these rocks, perthitic microcline (flake and veined perthite) predominates over pla- gioclase and quartz. Plagioclase in contact with microcline usually exhibits drop-like quartz inclusions, and myrmekite, and is often deformed with bent twinning plane and wavy extinction. Among the mafic phases, ferro edenite is the most common, sometimes forming agglomerates, showing twin-
Fig. 5. Magmatic structures in high-K calc-alkalic granitoids of the Transversal Zone: (a) snail structure; (b) ladder dike; (c) channel of pillow-like K-diorite in a more felsic matrix; (d) snapshot of a structure in a vertical section across a thermal plume head
- Estruturas magmáticas em granitóides cálcio-alcalinos de alto K da Zona Transversal: (a) estrutura em caracol; (b) diques em escada;
(c) canal de K-dioritos com estrutura em almofada em matriz mais félsica; (d) estrutura de uma seção vertical do topo de uma pluma tér- mica
a
b
c
d
that is 580-570 Ma old and another one that is 520 Ma old, and will be discussed in the next section.
The older group consists of ultrapotassic syenitoids that occur within the southwestern region of the terrane (Serra do Man batholith, Ferreira et al., 2002), or are part of the syenitoid line recognized by Ferreira & Sial (1986), which occur along the boundary between the Alto Pajeú and Cachoeirinha terrane (Triunfo, Serra do Casé, Serra do Livramento, Serrote das Duas Irmãs, Serrote do Paulo, and Bom Nome plutons). The ultrapotassic alkali feldspar syeni- toids are mineralogically very simple, consisting of perthitic microcline and aegirine-augite as main phases, and sphene, apatite, quartz, and rare magnetite and richterite after pyrox- ene, as accessory phases. Alkalic pyroxenites are found as co- magmatic inclusions, syn-plutonic, and late-stage dikes in the plutons along the syenitoid line, and have the same mineral phases as the host syenite, although in different proportions (Fig. 6). Host syenite and alkalic pyroxenite inclusions are interpreted as formed after liquid immiscibility process from a single mafic syenite magma (Ferreira et al., 1994).
Mica-pyroxenite xenoliths found in the Triunfo syenite are interpreted as derived from the lithospheric mantle source of the syenite (Ferreira et al., 1995). They are composed of diopside-salite (80% of the bulk rock), with subordinate (and in variable amounts) F-rich phlogopite, which shows chem- istry comparable with that observed in kimberlites and high magma chamber at short distances. The concentric struc-
tures represent magma flow in conduits and a frozen snap- shot of thermal plume. Mafic-intermediate syn-plutonic dikes, indicative of mingling of magmas, are also present.
Ages of plutons of this type range from 651 ± 15 Ma (TIMS zircon U-Pb, Tavares pluton, Brito Neves et al., 2003) to 638 ± 5 Ma (TIMS zircon U-Pb, Brejinho pluton;
Guimarães & Silva Filho, 2000). A Rb-Sr isochron yields an age of 638 ± 29 Ma for the Conceição das Creoulas pluton (Brasilino et al., 1998). Pb-Pb zircon evaporation method indicates an age of 643 ± 2 Ma for the Remédios pluton.
The plutons in the Alto Pajeú terranes are interpreted, on the basis of geochemistry, as emplaced in an early- to syn-orogenic event in a collisional setting (e.g.Guimarães et al., 1998; Brasilino et al., 1998; Brito Neves et al., 2003).
The plutons in the Cachoeirinha terrane are interpreted as emplaced syn-tectonically in a contractional event (Sial, 1993).
590-570 Ma-old, late-kinematic magmatism
The magmatic epidote-free high-K calc alkalic granitoids Plutons of this type are intruded in the Granjeiro ter- rane (Bodocó, Itaporanga and Serra da Lagoinha plutons that constitute the so-called Itaporanga-type granitoids as described by Almeida et al., 1967), and in the Alto Pajeú ter- rane (e.g. Fazenda Nova, Serra da Japecanga, Brejo da Madre de Deus, Campina Grande, Esperança batholiths).
They have been emplaced near terrane boundaries, and have peak emplacement age around 580 Ma, except for the Bezerros batholith, which is the oldest (687 Ma), and the only one that presents solid-state foliation (Silva et al., 1996).
Similarly to the mEp-bearing calc-alkalic granitoids, these plutons are characterized by porphyritic textures, with up to 8 cm long K-feldspar megacrysts. They range in com- position from quartz monzonite, quartz syenite to monzo- granite, forming a trend in the QAP diagram in the field of high-K calc alkalic granitoids (Fig. 2). As in the case of the the mEp-bearing HKCA granitoids, these plutons feature magma mingling with dioritic magma, and among these, a mixing of magmas of contrasting compositions, which was an important petrogenetic process (e.g. Mariano, 1989, McMurry, 2001, Neves and Vauchez, 1995, Sial et al., 1989). Evidence of successive mafic magma supplies, which continued at a declining rate, can be observed in the Fazenda Nova/Serra da Japecanga complex. This led to the production of stromatic-like structures, followed by the intrusion of contorted syn-plutonic dikes, and of late-stage dikes, some of which have chilled finer-grained margins (Neves and Vauchez, 1995).
Mixing of granitic and dioritic magma was more intense in some plutons (e.g. Itaporanga and Bodocó) and favoured the expressive development of hybrid granodior- ites showing field and petrographic evidence of mingling, such as pillow-like and net-veined structures, and mantled feldspar megacrysts captured from the host quartz mon- zonite. Large mixed zones can be mapped in these plutons at small scales (e.g.Mariano, 1989; McMurry, 2001).
The Itaporanga pluton in the Granjeiro terrane had its emplacement in a transpressive shear zone system, with the ascent of magma facilitated by pre-existing steep fault zone that bounds crustal discontinuities (Archanjo et al., 1999).
The peralkalic potassic to ultrapotassic syenitoids The Transversal Zone is the domain in the Borborema province that presents the largest and more widespread peral- kalic syenitoid and granitoid plutons. Syenitoids form a group
Fig. 6. Field relationships between the 570 Ma Triunfo ultrapotassic syenites and coexisting alkalic pyroxenite, formed by a liquid immisci- bility process (a) co-magmatic alkalic pyroxenite enclaves; (b) syn-plu- tonic dike of alkalic pyroxenite - Relações de campo entre os sienitos ultrapotássicos de Triunfo de 570 Ma e piroxenitos alcalinos coexistentes, relacionados por um processo de imiscibilidade de líquidos: (a) enclaves co-magmáticos de piroxeni- to alcalino; (b) dique sin-plutônico de piroxenito alcalino
a
b
pressure xenoliths in kimberlites, calcite, hyalophane, sphene, apatite, and traces of barite.
A gravimetric study performed on the Triunfo batholith (Motta, 2000), the largest known peralkalic ultrapotassic body in the Borborema province, confirms the existence of a denser layer, below the syenite, with a density compatible with that of alkalic pyroxenite. The gravimetric modeling suggests a proportion of 97.5/2.5 (syenite/pyroxenite), a lat- eral extension of 20 km and maximum thickness of 8.5 km, the pyroxenite being only 200 m thick, and a structure inter- mediate between a lacolith and a lapolith (Motta op cit.).
Magnetic fabrics and microstructures in the Triunfo batholith suggest that it has a tabular shape, and that magma emplacement occurred by lateral migration along a flat-lying crustal structure after fracture propagation and ascension of magma, during a crustal extension event with a minimum compressive stress (Archanjo & Bouchez, 1997).
Peralkalic potassic granitoids are quartz alkali syen- ites to alkali feldspar granites that occur as ring dikes around mEp-bearing trondhjemites in ring structures (Serrita and SW Serrita stocks; Neves; 1987; Ferreira & Sial, 1987); as a dike set near Catingueira town, and as small stocks (Guandu, Cavalos, Bernardo Vieira, and Cana Brava) (Ferreira & Sial, op cit). Only one pluton of this type has been dated (U-Pb zir- con age of 573 ± 45 Ma for the Catingueira pluton; Brito Neves et al., 2003). These rocks contain aegirine-augite and alkalic amphibole as major mafic phases, with magnetite and apatite as main accessory phases. Fluorite and cassiterite are locally present at amounts < 0.5%, and albitization is locally an important phenomenon (e.g. ring dikes at Serrita stocks;
Ferreira & Sial, op cit.).
The metaluminous high-K syenitic plutons
Plutons with these characteristics are found in the Cachoeirinha (Terra Nova pluton), Alto Pajeú (Pajeú) and Rio Capibaribe (Toritama and Bom Jardim plutons) terranes.
The 150 km2Terra Nova pluton is mainly composed of porphyritic syenite and quartz syenite in which hornblende and feldspars are up to 2 cm long phenocrysts in a matrix composed of hornblende, plagioclase, quartz, sphene, biotite, epidote, and ferro-augite (Silva Filho et al., 1987). The emplacement of this pluton, at 556 ± 79 Ma (Rb-Sr; Silva Fig. 7. Photomicrographs of 545-520 Ma peralkalic dikes of the
Transversal Zone: (a) richterite-bearing syenite (largest crystal is c. 0.5 mm long) (nicols not crossed); (b) aegirine-augite syenite (pyroxene crystal is ca. 0.4 mm across) (nicols crossed)
- Fotomicrografia de diques peralcalinos de 545-520 Ma da Zona Transversal: (a) sienito com richterita (cristal maior tem cerca de 0,5 mm de comprimento) (nicóis paralelos); (b) aegirina-augita sienito (cristal de piroxênio tem cerca de 0,4 mm de comprimento (nicóis cruzados)
Fig. 8. SiO2vs. K2O diagram for Transversal Zone grani- toids and syenitoids. Fields are from Peccerillo and Taylor (1976). SH= shoshon- ite, HK = high-K calc-alkalic, MK = medium-K calc-alka- lic, Low-K = low-K calc alka- lic fields.Epi = epidote;
HKCA = high-K calc-alkalic;
SH = shoshonitic; MHK = metaluminous high-K;
Trondh = trndhjemite; CA = calc-alkalic
Fig. 8. Diagrama SiO2vs.
K2O para granitóides e sieni- tóides da Zona Transversal.
Os campos são de Peccerillo and Taylor (1976). SH=
shoshonito, HK = cálcio- alcalino de alto K, MK = cál- cio-alcalino de médio K, Low-K = cálcio-alcalino de baixo K. Epi = epidoto;
HKCA = cálcio alcalino de alto K; SH = shoshonito;
MHK = metaluminoso de alto K; Trondh = trond- hjemito; CA = cálcio alcalino
b
a
migmatites of the Alto Pajeú terrane. They comprise mafic syenite-monzonites, mesocratic syenites and leucosyenites in which mafic syenites occur as enclaves and synplutonic dikes (Guimarães & Silva Filho, op cit.). These authors interpreted the complex as intrusive into the later stage of the Brasiliano cycle and magmas generated in a metasomatized lithospheric mantle.
520 Ma-old, post-orogenic magmatism The A-type plutons
Two calc-alkalic A-type plutonic complexes have been recognized in the Transversal Zone. One of which is the 570 Ma Queimadas foliated pluton (Almeida et al., 1997). The other one, which is better known, is the 520 Ma-old Prata complex that constitutes a 250 km2 composite intrusion in gneisses and migmatites (Melo et al., 1996), Alto Moxotó ter- rane, next to the boundary with the Alto Pajeú terrane. It com- prises several granitic intrusions, and dioritic dikes. The main pluton consists of porphyritic coarse-grained biotite horn- blende monzonites and medium-grained equigranular biotite sienogranites, in which rapakivi texture occurs locally.
Fluorite is a common accessory phase, a typical mineral of A- Filho et al., 1993) is interpreted to have occurred during late-
collisional stage of the Brasiliano cycle, controlled by move- ments of the Pernambuco shear zone, in a compressive regime (Silva Filho et al., 1993).
The Pajeú syenite is a large (40 km vs. 20 km) elon- gate pluton intrusive into gneisses and migmatites of the westen part of the Alto Pajeú terrane, presenting two major facies: porphyritic syenite, the major facies, characterized by 2-8 cm K-feldspar megacrysts, and fine-grained syenite. Mineral phases are the same in the two facies (microcline, orthoclase, quartz, minor plagioclase, calcic amphibole, sphene, epidote, allanite, apatite, zircon, magnetite) (Reyes et al., 1996).
Melanocratic syenitic enclaves, which present acicular apatite, and net-veined structures, and synplutonic dikes, are indicative of mingling, and locally mixing of magmas.
Enclaves are aligned following the magmatic foliation. TIMS U-Pb zircon age of 586 ± 21 Ma is reported by Guimarães &
Silva Filho (1998). Reyes et al. (op cit.), on the basis of geo- chemical data, interpret that the emplacement of this pluton took place in a late to post-collisional setting.
The Bom Jardim (Rb-Sr age of 585 ± 38 Ma; Guimarães and Silva Filho, 1997) and Toritama complexes are elongate multiple-injected magmas intrusive into gneisses and
Terrane Rio Capibaribe Rio Capibaribe Alto Pajeú Alto Pajeú Alto Pajeú Alto Pajeú Alto Pajeú Alto Pajeú Cachoeirinha Cachoeirinha Granjeiro Granjeiro Rio Capibaribe Rio Capibaribe Rio Capibaribe Rio Capibaribe Alto Pajeú Alto Pajeú Alto Pajeú Alto Pajeú Alto Pajeú Alto Pajeú Alto Pajeú Alto Pajeú Cachoeirinha Cachoeirinha Cachoeirinha Cachoeirinha Alto Pajeú- Cachoeirinha Pajeú boundary Alto Pajeú- Cachoeirinha Pajeú boundary Alto Pajeú- Cachoeirinha Pajeú boundary Alto Pajeú- Cachoeirinha Pajeú boundary Alto Pajeú- Cachoeirinha Pajeú boundary
Magma series HKCA HKCA HKCA SH HKCA HKCA SH HKCA CA CA HKCA HKCA MHKS HKCA PER/SH PER HKCA SH HKCA MHKS UP UP A-type TROND CA SH MHKS PER PER
PER UP
Pluton Bezerros Timbaúba Tavares Teixeira Brejinho
Conceição das Creoulas Serra do Arapuá Remédios Conceição Emas Itaporanga Bodocó Bom Jardim Fazenda Nova Pinheiro Moderna
S-Caldeirão Encantado Betânia
Campina Grande Pajeú Serra do Man Triunfo Prata Palmeira Angico Torto Salgueiro Terra Nova pluton Catingueira Terra Nova dikes
Princesa Isabel dikes
Serra do Livramento
Serrote de Duas Irmãs
Bom Nome
Age Ma 687 ± 34 644 ± 5 651 ± 15 645 638 ± 5 638 ± 29 612 ± 24 643 + 2 635 ± 9 / 633 ± 9 627 ± 19 585 ± 2 559 - 593 592 ± 7 / 585 ± 38 588 ± 12
581 ± 1.6 586 ± 21 592 ± 49 581 ± 28 572 512 ±30 505 ± 4 557 ± 71 556 ± 79 573 ± 45 515 ± 20
Method Pb-Pb U-Pb U-Pb U-Pb U-Pb Rb-Sr Rb-Sr Pb-Pb U-Pb / Rb-Sr U-Pb U-Pb Rb-Sr U-Pb U-Pb
U-Pb U-Pb Rb-Sr Rb-Sr Rb-Sr Rb-Sr U-Pb Rb-Sr Rb-Sr U-Pb Rb-Sr
(87Sr/86Sr)i
0.70526 0.7115-0.7080 0.7093 0.71054 0.70598 0.70911 0.7058 0.7057- 0.7063 0.70714
0.70839
0.70903 0.71087 0.710 0.7132 0.71062 0.7047 0.0761 0.71071 / 0.70978
0.7098
0.7088
0.7106
εNd(0.60)
-4.2 to -5.4 -3.4 -13.9 -3.6 to -10.1 - 2.8 -16.6 -1.2 -2.0 -9.0
-19 -12.4 -7.1 -7.6 -13.6 -5.0 -11.9 -16.5 -18.6 -16.0 -17.3 -16.1 -14.2 -3.8 -13.5 -15.6 -17.5
-3.6
T DM 1.4 1.4 2.1 1.84 -1.42 1.31 2.14 1.23 1.30 1.65
1.9 1.92 1.92 2.14 1.5 1.87 2.10 2.07 2.17 2.47 2.4 2.17 1.40 1.86 2.39 2.37
1.38
reference Silva et al., 1996 Guimarães et al., 2001
Brito Neves et al., 2003*, this work#
BBBN oral commu.*; Ferreira, 1991#, this work**
Guimarães e Silva Filho, 2000*, Torres, 2000#
Brasilino et al.,**, Brasilino 2003#
This work
This work##, Lins, 2000#
Brito Neves et al., 2003*, Sial, 1993*#
Van Schmus et al., 1995#, This work*/**
Brito Neves et al., 2003*, Sial 1987**, Van Schmus et al., 1995#
McMurry, 2001
Guimarães and Silva Filho, 1997 Van Schmus et al., 1995 This work
This work This work Lins, 2000
Almeida et al., 1997*, Guimarães et al., 1998#
Guimarães and Silva Filho, 1998 Ferreira et al
Ferreira et al, 1995 Melo et al., 1996 Kozuch et al., 1997 This work
Silva Filho, et al., 1990**, Van Schmus et al., 1995#
Silva Filho et al., 1993**, Van Schmus et al., 1995#
Brito Neves et al., 2003*, Van Schmus et al., 1995#
Silva Filho and Guimarães, 1990**; Ferreira and Sial, 2002
Ferreira and Sial
Ferreira, 1991
Ferreira, 1991
Ferreira, 1991
Table 1. Radiogenic isotope data for granitoids from the Transversal Zone, Borborema Province, NE Brazil. On the references: * U-Pb data,
** Rb-Sr, data # Sm-Nd, Data## Pb-Pb, data. Group of granitoids: HKCA = high-K calc alkalic; MHKS = metaluminous high-K syenitoids, PER = peralkalic, SH = shoshonitic, UP = ultrapotassic
type granitoids emplaced at high levels into the crust. Field evidence for mingling with dioritic magmas, such as syn-plu- tonic dikes, and net-veined and pillow-like structures, are observed in most part of the pluton. This complex is inter- preted by Melo et al. (op cit.) as the result of an extension- related crustal anorogenic magmatism.
Peralkalic syenitic dikes
The second group of peralkalic rocks as referred to in section “The peralkalic potassic to ultrapotassic syenitoids”
consists of potassic, silica-saturated syenitoids intrusive into low-metamorphic grade metapelites of the Cachoeirinha ter- rane (Riacho de Santo Antônio dikes, Ferreira & Sial, 2002), at the Rio Capibaribe terrane (Moderna and associated dike,
Ferreira & Sial, op cit) or forms two dike sets, intrusive at the southernmost Cachoeirinha terrane, next to the boundary with the Alto Pajeú terrane (Terra Nova dikes) or across the bound- ary of the two terranes (Princesa Izabel dikes) (Silva Filho et al., 1993, Ferreira & Sial, 1997). Each dike set is composed of about 50 dikes each, and are texturally and mineralogical- ly similar.
They are silica saturated to slightly oversaturated alkali feldspar trachyte/syenite, presenting large textural vari- ation, including aphanitic, fine- to medium-grained types, which have equigranular, inequigranular or microporphyritic textures. Main phases in both dike sets are zoned orthoclase, aegirine-augite to aegirine and amphibole (early crystallized pargasite to Fe-pargasite or late-crystallized ferro-richerite) Fig. 9. Shand index diagram for Transversal Zone granitoids and syenitoids. Symbols are the same as for Fig. 8
- Diagrama de índice de Shand para granitóides e sienitóides da Zona Transversal. Os símbolos são os mesmos da Fig. 8
Fig. 10. R1 vs. R2 diagram (Batchelor and Bowden, 1985) for Transversal Zone granitoids and syenitoids. Symbols are the same as for Fig. 8
- Diagrama R1 vs. R2 (Batchelor & Bowden, 1985) para granitóides e sienitóides da Zona Transversal. Os símbolos são os mesmos da Fig. 8
tallization as major petrogenetic process, in spite of the field evidence for an important mixing and assimilation process of more mafic magma in the high-K calc alkalic plutons. The metaluminous character of these two suites is compatible with generation from anatexis of a hornblende-rich source (eg. Whitney, 1988), probably in the lower crust. Higher Rb and Sr contents, and higher Rb/Sr ratios for the HKCA gran- itoids than for other suites are compatible with the presence of coexisting, although volumetrically small, mafic magmas.
The mEp-bearing shoshonitic rocks (SH) are metalu- minous in composition (Fig. 9), exhibit a relatively wide range of SiO2(from ca. 60 up to 75% SiO2), and are characterized by lower MgO, TiO2, Fe2O3T, and P2O5 contents, higher Al2O3and Na2O, and higher Ba/Rb and Sr/Rb ratios than the other groups. They present decreasing Na2O and Al2O3with increasing silica contents, in contrast with the other series in which these oxides have positive correlation (Na2O) or remain constant (Al2O3) with increasing silica.
The potassic and ultrapotassic (UP) plutons are peralka- lic (Fig. 9), characterized by enrichment in incompatible ele- ments (K, Sr, Ba, P, REE), have low Zr and Nb concentrations, and present Nb and Ti negative anomalies in MORB-normal- ized diagrams (Ferreira, 1991). Enrichment in these elements in the ultrapotassic syenites, as well as in their alkalic pyroxen- ite co-magmatic inclusions are interpreted as inherited from a metasomatized-mantle source. This hypothesis is reinforced by the chemistry of the mica-pyroxenite xenoliths that show also high K and Ba contents (K2O = 1-5wt%; Ba = 1400-1500ppm), variable Cr (21-3000 ppm) and Ni (20-280ppm), very low Nb (6 ppm), and high F (0.4-1.8 wt%) contents, being LREE- enriched (Ferreira et al., 1995).
The trondhjemitic granitoids are all peraluminous, have a major chemistry similar to that of continental trondhjemites, with high Al2O3contents (usually > 16%), low MgO (0.10 to 0.53%) and CaO (0.33 to 1.7%), and predominance of Na2O over K2O (Na2O/K2O varies from 1.19 to 2.6) (Neves, 1986).
Alumina contents decrease with increasing silica contents, and together with very high Sr (1800-3700 ppm) and Ba (1800 to 7400 ppm) contents, indicate early crystallization of (Fig. 7). Amphibole in the Terra Nova dikes has more variable
compositions, and has crystallized straight from the magma, while in the Princesa Isabel dikes, amphibole was largely pro- duced after pyroxene, with few exceptions. Larger textural variation is observed in the Terra Nova dikes, which cut, among other rocks, the peralkalic ultrapotassic Serra do Casé and Serra do Livramento plutons (570 Ma), attesting their younger ages (Rb-Sr age of 515 ± 20 Ma; Silva Filho et al., 1993).
On the basis of marked differences in magnetic suscep- tibility, O and Nd isotopic data for the two dike sets, Ferreira
& Sial (1997) concluded that they are derived from different mantle sources. The Princesa Izabel dike set is interpreted as emplaced in a post-orogenic, extensional setting, and marks the amalgamation after collision between the Cachoeirinha and Alto Pajeú terranes. The emplacement of the Terra Nova dike set is interpreted as controlled by the strike-slip regime of the Pernambuco shear zone in the Early Cambrian (Silva Filho et al., 1993).
Geochemistry and tectonic setting
Magmatic epidote-bearing granitoids from all series group close to each other in a SiO2vs. K2O diagram, in the HKCA field of Peccerilo & Taylor (1976), with a few samples lying in the calc-alkalic and shoshonitic fields (Fig. 8).
Samples from the other series plot in the shoshonitic field in this diagram. Trondhjemitic plutons are medium- to high-K calc-alkalic, peraluminous in composition.
The high-K calc alkalic (HKCA), both magmatic epi- dote bearing and free, and calc alkalic (CA) granitoids are metaluminous to peraluminous (Fig. 9), being undistinguish- able from each other on the basis of major and minor ele- ments, as they form overlapping trends in most variation dia- grams. They show negative correlations between SiO2 and TiO2, CaO, MgO, MnO, and Fe2O3, the opposite behavior being observed with Na2O and K2O, while Al2O3 remains constant with increasing silica. Positive correlations between Ba/Rb vs. Zr/Rb, and Zr/Rb vs. Sr/Rb attest to fractional crys-
Fig. 11. log Rb vs. Log (Y + Nb) diagram of Pearce et al.(1984) for Transversal Zone granitoids and syenitoids. Syn-colg = syn-collisional granites; WPG = within-plate granites; VAG = volcanic-arc granitoids. Symbols are the same as for Fig. 8 Diagrama log Rb vs. Log (Y + Nb) de Pearce et al. (1984) para granitóides e sienitóides da Zona Transversal. Syn-colg = granites sin-col- isionais, WPG = granites intra-placas, VAG = granites de arco vulcânico. Os símbolos são os mesmos da Fig. 8
plagioclase, which is reflected in the peraluminous nature of the granitoids. Low Rb/Sr precludes the use of the Rb-Sr sys- tematics for dating (Neves, op cit.). This author proposed that the Serrita stock was generated through partial melting of altered basaltic rocks, where amphibole and garnet remain in the residue, and subsequent crystallization at pressures above 6 kbar.
The high-K syenitoids are metaluminous to slightly peraluminous (Fig. 9), have SiO2contents varying from ca.
55 to 65wt%, plotting in the field of ultrapotassic rocks in the SiO2vs. K2O% diagram (Fig. 8). Likewise the peralkalic plu- tons, these syenitoids do not show Eu anomaly in REE chon- drite-normalized diagrams. Guimarães & Silva Filho (1990) found high Ba (2000-4500 ppm) and very high Sr (1000-2200 ppm) contents, as well as high Cr (up to 470 ppm) and Ni (30- 250 ppm) in the Bom Jardim complex, compatible with man- tle-derived magmas, with minor crustal contamination.
Figures 10 and 11 show tectonic discrimination dia- grams for the studied suites of rocks. The oldest (650-620 Ma) plutons display signatures typical of syn-tectonic mag- matism (destructive plate margins and volcanic-arc granites).
On the other hand, the 590-570 and 545-520 Ma plutons are similar to late to post-tectonic granites (late orogenic and vol- canic arc granites transitional to within plate granites).
Sr and Nd isotopes
Sr and Nd isotopic data for plutons of the Transversal Zone compiled from literature and from this work are listed in Table 1.
The initial (87Sr/86Sr) ratios of the shoshonitic grani- toids are the lowest among the studied granitoids, varying from 0.7047 (Salgueiro pluton) to 0.70526 (Teixeira grani- toid), compatible with limited crustal contribution. Initial Sr ratios for the mEp-bearing CA granitoids are variable, rang- ing from 0.70598 to 0.710621. These values are lower,
although partially overlapping, than the values for the mEp- bearing HKCA group (0.70839-0.71002). Low initial Sr ratios are observed for the epidote-free HKCA granitoids, around 0.706, in spite of field evidence for local mixing between crustal and mantle-derived magmas. The A-type granitoid presents high initial 87Sr/86Sr (0.71320; Melo et al., 1996), which suggests an important crustal component involved in their source.
The peralkalic potassic and ultrapotassic syenitoids have high initial Sr ratios, 0.710, which together with strong- ly negative
ε
Ndvalues (ca. -19) are interpreted as inherited from a contaminated upper mantle source (Ferreira et al., 1997).The values of
ε
Nd(0.6 Ga) for all groups are negative, and there is a tendency for younger suites to have more neg- ative values, compatible with interaction with a thicker crust.Magmatic epidote-bearing CA granitoids from the Cachoeirinha terrane display
ε
Nd (0.6Ga) in the -1 to -2 range, and TDM model ages between 1.2 and 1.4 Ga (Sial et al., 1999). These data, together with the low initial Sr ratios and widespread presence of amphibole-rich clots, are com- patible with the hypothesis of generation of these tonalites and granodiorites by partial melting of a metabasalt.Values of
ε
Ndfor these mEp-bearing CA granitoids are lower and with a restricted range compared to the values for the mEp-bearing HKCA granitoids that occur in the Alto Pajeú terrane (-2.8 to - 10.1). The values ofε
Nd are morevariable also in a single pluton (e.g. from -3.6 to -10 in the Brejinho granitoid) and are compatible with variable degrees of interaction of mantle-derived magma with crustal material.
The peralkalic syenite dikes present two distinct
ε
Nd and TDMvalues: less negativeε
Nd(-3.6) and younger TDM (1.38 Ga) for the Princesa Izabel dikes, that occur across and in both sides of the Cachoeirinha and Alto Pajeú terrane boundary, at the northernmost part of the syenitoid line, and more negativeε
Nd(-17) and older TDM (2.37 Ga) for the Fig. 12. Nd vs. Nd model age plot for Transversal Zone granitoids and syenitoids. Symbols are the same as for Fig. 8- Diagrama Nd vs. idade modelo Nd para granitóides e sienitóides da Zona Transversal. Os símbolos são os mesmos da Fig. 8
ships that show close relationships between the host granitoid with K-dioritic magmas. An alternative hypothesis is that the oldest granitoids are the product of remelting of Mesoproterozoic continental crust.
Most works on the Borborema province have conclud- ed studies that during the Brasiliano cycle no juvenile materi- al was added to the crust, although mantle material is very common and presence of two magmas is observed in most suites of the region (Sial et al, 1992). A tectono-thermal evo- lution with intralithospheric differentiation is interpreted for the Brasiliano cycle at this part of the Borborema province (Neves et al., 2000). Positive
ε
Nd isotopic data determined on calc-alkalic granodiorites emplaced at the easternmost extension of the Patos shear zone (B.B. de Brito Neves, writ- ten communication, 2003), however, suggest presence of juvenile material. This evidence, although observed so far in only one Neoproterozoic pluton, coupled with the possibility that juvenile Neoproterozoic mantle-derived magma was involved in the production of the 650-620 Ma granitoids, must be taken into account in the models for the tectonic model for the evolution of the province.The 590-570 Ma plutons have Sr and Nd isotopic values of magmas formed by remelting of Paleoproterozoic lower continental crust, the only geochemical reservoir with these characteristics. The only exception among these younger rocks are the ultrapotassic peralkalic syenitoids, which present high initial Sr ratios and very negative
ε
Ndvalues, interpreted as derived from an incompatible element enriched-mantle source.
The 545-520 Ma peralkalic dikes present Paleoproterozoic Nd model ages, but the less negative
ε
Nd values for the Manaíra-Princesa Izabel dikes suggest smaller degree of con- tamination, probably during ascent and emplacement at low pressure, as also suggested by oxygen isotope values ( δ18O from +8 to +9‰ SMOW; Ferreira & Sial, 1997) for these rocks. Strongly negativeε
Ndvalue for the Terra Nova dikes suggest an incompatible element enriched mantle source, similar to that for the syenites along the syenitoid line.Acknowledgments
Part of the field and laboratory work was made pos- sible by means of a grant from CNPq to VPF process n.
463029/00-3.
Terra Nova dikes, that occur in the Cachoeirinha terrane, next to the boundary with Alto Pajeú terrane, at the southernmost syenitoid line.
Depleted mantle Nd model ages tend to be older in the younger suites. The mEp-bearing CA and HKCA grani- toids from the Cachoeirinha and Alto Pajeú terranes have source of Mesoproterozoic age, while the other suites, includ- ing the mEp shoshonitic granitoids have Paleoproterozoic model ages (Fig. 12).
Discussion and conclusion
Predominance of magmatic epidote-bearing granitoids (solidification pressures around 6 kbar) in the 650-620 Ma interval in contrast with the presence of magmatic epidote- free (solidification pressure around 3-4 kbar) in the 590-570 Ma time interval for magmas of similar composition is prob- ably due to contrasting oxidation state of the magma, and mode of emplacement to different stages of tectonic evolu- tion. Late collision is characterized by a gradual change of motion from perpendicular to the collision zone to transcur- rent motion, which is initially achieved by ductile deforma- tion, while the collision is still hot, and is superseded by brit- tle fracturing (Shackleton, 1986). Hot crust and ductile defor- mation would allow magma to have a rather fast emplace- ment and solidification rate, which prevented epidote dissolu- tion (e.g. Sial et al., 1999), as compared with epidote-free similar suites emplaced later in the Brasiliano cycle (ca. 580 Ma), in a post-collisional, extensional setting, in brittle con- ditions, and in which thicker crust was formed.
The 650-620 Ma old granitoids present younger
ε
Ndmodel ages (< 2.0 Ga) and less negative Nd (ca. -2 to -14) than the 590-570 Ma granitoids, for which TDMvaries from ca 1.5 to 2.5 Ga, and
ε
Ndfrom ca. -8 to -20. The highest and more variable initial 87Sr/86Sr values, on the other hand, are observed in the oldest granitoids, while lower initial Sr ratios are found in the 590-570 Ma magmas. These data indicate an important change of type of magma source with time. The oldest granitoids could represent a mixture of Neoproterozoic, juvenile, mantle-derived material with magma derived from melting of Paleoproterozoic continental crust, which would yield intermediate Nd model age, of ca.1.2-1.4 Ga. This is compatible with mixture of mantle- derived and crustal magmas, as indicated by field relation-
